US2655043A

US2655043A - Pressure measuring device

Info

Publication number: US2655043A
Application number: US32888A
Authority: US
Inventors: Charles M Wolfe; Quido M Shultise; Clyde W Grow
Original assignee: Aerojet General Corp
Current assignee: Aerojet Rocketdyne Inc
Priority date: 1948-06-14
Filing date: 1948-06-14
Publication date: 1953-10-13
Anticipated expiration: 1970-10-13

Description

1953 c. M. WOLFE ET AL 2,655,043

PRESSURE MEASURING DEVICE Filed June 14, 1948 2 Sheets-Sheet l y LYDE W. GROW A TTOR/VEY Oct. 13, 1953 c. M. WOLFE ET AL 2,555,043

PRESSURE MEASURING DEVICE Filed June 14. 1948 2 Sheets-Sheet 2 I fiAX /V A ---1 I OSCLLATOR I73 IE-{AT ,Z l r *5 I 36 r 1 33 if: a: i E

35 T i L i F1 5 INDICATOR AMPUFIER T J' ERF {Ly V O 1 F17. 11 N 575 5 ifi D. L 4 /Q47M/ ATTORNEY Patented Oct. 13, 1953 PRESSURE MEASURING DEVICE Charles M. Wolfe, Glendora, Quido M. Shultise,

Pasadena, and

Clyde W. Grow, Sierra Madre,

Califl, assignors, by mesne assignments, to

Aerojet- General Corporation,

a corporation of Ohio Application June 14, 1948, Serial No. 32,888

2 Claims. (01. 73-398) This invention relates to electrical pressuredetecting systems, and more particularly to arrangements of this character which are adapted for measurement of rapidly varying pressure and other phenomena of a similar nature.

Objects of the invention are to provide a displacement-detecting arrangement of the varying capacitance type which is of a relatively small, compact and simple design and is easily usable.

Displacement detecting or pickup devices and systems have heretofore been proposed for measuring variations of pressure such as occur in a device like a jet propulsion motor operated by intermittent explosions. Such applications of a pickup pose problems due to wide variations in rapidly varying pressure both above and below atmospheric, and also due to high temperatures. It is desired furthermore that a pickup device for such a purpose be extremely sensitive, and be capable of responding quite accurately to pressure variations over a Wide range of intensities and frequencies under the adverse conditions of measurement.

In accordance with our invention, we provide a pickup or pressure detecting device and system especially adapted for the above purposes.

Our pickup comprises a variable condenser formed by variation of capacity due to a pressure-responsive diaphragm; and the pickup is incorporated in a system comprising a radio frequency oscillator, the output of which is applied to the pickup circuit. The alternating voltage on the variable condenser is modulated in conformance with displacement of the condenser diaphragm. This modulated radio frequency is rectified in a rectifier, and the rectified output is sent to an indicating or recording device such as an oscillograph, preferably through a low gain amplifier.

Preferred features of the system comprise the use of a pad of resistors between the variable capacity of the pickup and the oscillator, for the purpose of matching the impedance of the pickup with that of the line from the oscillator.

An important feature of the pickup construction resides in a compact arrangement of a rectifier and resistor associated with the pickup capacity and the rectifier.

The foregoing and other features of our invention will be better understood from the following detailed description and the accompanying drawing, of which:

Fig. 1 is a side elevation view of a pickup unit according to our invention;

Fig. 2 is a view of the pressure responsive end of the pickup unit;

Cincinnati, Ohio,

Fig. 3 is a cross-section view of the unit taken on line 3-3 of Fig. 2;

Fig. 4 is a view taken on the line 4-4 of Fig. 3;

Fig. 5 is a cross-section view taken on the line 5-5 of Fig.3;

Fig. 6 is a view schematically showing the electrical circuit of the pickup unit proper;

Fig. 7 is a side elevation view of the cable retainer;

Fig. 8 is a cross-section View of the cable retainer taken on the line 8-8 of Fig. 7;

Fig. 9 is a schematic wiring diagram of the entire electric system employing the pickup unit of Figs. 1 to 8;

Fig. 9a is a schematic wiring diagram of a modified form of wiring diagram which can be substituted for a corresponding portion of the diagram of Fig. 9;

Fig. 10 is a vector diagram for the circuit; and.

Fig. 11 is a curve showing the rectified out put voltage vs. time.

Referring to the drawing, the pickup comprises a housing I which is of a generally cylindrical shape having integral with it a peripheral flange 2 back of the front end portion 3. A number of bolt holes 4 are provided around the flange 2, so that bolts may be passed through them to bolt the pickup to the wall of the chamber or device in which the pressure is to be measured. For example, it may be desired to measure the changes of pressure within the combustion chamber of a jet motor or aeroresonator or the like; in which case, a hole may be provided in the wall of the combustion chamber large enough to insert the front end 3 of the pickup and to bolt the flange 2 to the chamber wall. At the front end of the pickup there is provided a variable condenser one plate of which is adapted to displacement in accordance with variations of pressure. This comprises a diaphragm 5 fastened at the front end; and provision is made for fastening it, comprising a reentrant peripheral flange 6 integral with the diaphragm and adapted to register with a corresponding reentrant flange 1 formed around the periphery of the forward housing.

The circular diaphragm portion 5 is made sufficiently thin and flexible and designed so that the displacement of the diaphragm is linearly proportional to the differential pressure. The diaphragm is held in its forward position against the flange 1 by means of a cylindrical sleeve 8 having a forward peripheral flange 9 which abuts a flange 6; and the rear end of sleeve 8 extends out to the rear end of cylindrical housing I and preferably just a slight amount beyond, so that the flange 9 and is held in the forward position by an insulating member I3 which extends across the cross-section of the pickup. Insulator I3 is integral with insulator portion I5. The

insulators I3 and I5 may conveniently bemade i of a hard rubber or the like. Insulating cylinders I3 and I5 may also be replaced by a metal cylinder of similar construction provided with sufficient clearance about the element connec: tions of R1, R2 R3 and 33 to permit electrical insulation. The advantage of the use of the brass cylinder is that a high degree of electrical shielding is insured between the circuit elements. This can also be accomplished by metal plating certain surfaces of insulator I3 and I5.

At the rear of insulator there is placed a circular collar It. Adjacent to collar I9 there is a spider l6 which has a flange lea that contacts circular collar I9. Spider I6 is provided with arms I! which support a cylindrical sleeve I8 designed to fit within the sleeve of member III. The assembly of collar I5 and the sleeves I9 and I5 and the members ahead of it are held in position by an internal nut which threads into threads 21 formed at the rear of sleeve 8. Then after the nut 2I is tightened up to tighten the assembly, the rear member I0 is slid over the sleeve I8 and bolted in position.

There is provided immediately back of the diaphragm 5 an electrode 22 in the form of a button having a flat surface 23 parallel to the rear surface of the diaphragm 5 and spaced therefrom by a small amount to create an electrical capacity; and the button has a stem 24 which is threaded on both its inside and outside surfaces. The outer threads of shank 24 are threaded into a metallic sleeve 28 within the in sulator annulus I2. In order to form a secure engagement with the annulus, the forward portion of member 28 is provided with bevel 29 and the rear portion is provided with a shoulder 30 to engage corresponding surfaces of the insulator.

annulus. To bind the stem 24 within the sleeve 28, there is provided a threaded screw for threading into the internal threads of thestem; and to produce the binding action, these internal threads of the stem gradually reduce in diameter in the forward direction so that the binding action of the screw progressively increases as the screw penetrates further into the stem. The stem is a split stem so that its split walls will thus be expanded outwardly to bind .against sleeve 28. By this expedient the forward and rearward position of the condenser button 22, relative to diaphragm 5 may be adjusted to any desired position, and then bound in this position by tightening down the set screw 25, for example, by a set screw provided with a hexagon depression that may be tightened with a hexagon wrench.

In addition to the variable condenser element just described, the pickup also contains a rectifier element and resistor elements. To support these elements the cylinder I5 is provided with a number of bores 3I, arrangedaround the periphery,

and into which there are placed respective cartridge-type resistors, designated as R1, R2 and Rs, as indicated in Fig. 4. The cylinder [5 is also provided with a central bore or hole 32, into which is inserted a cartridge-type rectifier element 33, which is preferably a half-wave crystal rectifier of a type which is available in small cartridge form.

The arrangement of the electrical elements within the casing facilitates the interconnection of-the elements by wiring; and for this purpose suitable holes 44, 45, 45 and 4! are provided through the insulators through which wires from the cartridges may pass. Concentric lines 36 and 31 having their respective outer conductors 36a and- 3'Ia are clamped into the fitting I6 and connected to the proper elements of the pickup. Figs. 3, 4 and 5 indicate part of the wiring of the elem .ments; and although the wiring is not shown complete in these figures, a complete showing in this'manner is unnecessary, in view of the schematic wiring diagrams of Figs. 6 and 9, which indicate how the wiring is completed.

Fig. 9 is a schematic view of the electrical system in the pickup device just described incorporated in a complete system for indicating pres sure variations. The dotted rectangle 34 represents the pickup unit of Figs. 1 to '7; and the elements within the rectangle are the elements within the unit. The variable condenser C1 represents the variable capacity between diaphragm 5 and the button 23; the half -wave rectifier D represents the cartridge rectifier 33; and the re-, sistors R1, R2 and R3 represent the same desig-.-.

nated resistor elements in Figs. 3 and 4. It should be noted that the resistor R1 is actually shown as four resistors connected inrparallel, but

it will be understood that only a single resistor.

R1 of the proper value is required. The choice of whether to use one or more of these resistors in parallel will depend on convenience and ease of selection of values; and the over-all value of the parallel-arranged resistors should total the amount desired for R1 in Fig. 8. The elements within the rectangle 34 constitute a network of which one side is grounded, the ground being the casing of the pickup unit. The ungrounded side of the resistor R1 and of this network is connected to the output of a radio frequency oscillator 35 which should preferably be of the high frequency type within the range of about 5 to 100. megacycles per second; and the line 36 which.

couples the oscillator to the network should prefera-ply be of the coaxial type having its outer conductor grounded. The coaxial line may be of some well known type having a characteristic impedance, for example of about '72 ohms resistl anc 8.

so that it does not have an appreciable effect asa shunt across R1. For this purpose, R2 should be much larger than R1 and may conveniently have a value of about 1000 ohms when R1 is in the order of 72 ohms. The air gap and size of the elements of condenser C1 may conveniently be adjusted so that it has a capacity of about 20' mmf. at zero deflection.

The value of resistance R1 across which the rectified voltage is developed should preferably be sufficiently high as not to dissipate much of the rectifier energy and may have a value in theorder of about 10,000 ohms, where R1 and R: are of the order of 72 ohms and 1000 ohms respectively. The distributed capacitance of the cable 31, Fig. 9 in combination with resistance element R3 acts to filter the rectified voltage Em, and to minimize an radio frequency standing wave on cable 31. The resistance R3 can be located in the amplifier 38. A small fixed capacitance can be connected in the pickup to advantage, replacing resistance element R3. This small fixed capacitance might be of the order of 500 mmfd. and would greatly reduce the amplitude of any radio frequency voltage appearing on cable 31. It will be understood that the foregoing values of the electrical elements are given by way of illustration rather than of limitation, and that many other combinations of values can be made to operate satisfactorily.

The output of the rectifier is connected through a grounded line 37 to the input of an amplifier 38 of a, type adapted to amplify the rectified output, that is, the variations of the D. C. rectified voltage; and the amplified output is shown coupled to a suitable indicating device 39 which may, for example, be an oscillograph or oscilloscope.

Fig. 9a shows a modification of the portion of the system within the rectangle 34 which can be substituted for the corresponding portion in Fig. 9. The only change in Fig. 911 from the system of Fig. 9 resides in the substitution of a condenser C2 for the resistor R2 in Fig. 9. A good value for C2 will be such as will make its impedance about the same as that suggested for R2 above at the frequency of the oscillator. The condenser C2 can be made of a size and shape to fit conveniently in the housing of the pickup in place of R2.

In operation, the pickup unit will be attached in a conventional manner to the device such as a combustion or rocket motor chamber with the diaphragm end inserted into the chamber to receive the variations of pressure. The diaphragm will deflect in accordance with the pressure variations, to vary the capacity C1 accordingly. The Voltage conditions in the pickup are as shown by the vector diagram in Fig. 10, wherein the voltages ER2 and Ec are shown to add up vectorially to the voltage ER,, which is across resistor R1. Owing to the relatively low resistance of R1, this radio frequency voltage ER, is substantially constant. The voltage Ec, however, varies with the capacity variation with corresponding variation of voltage ER, across resistor R2. Accordingly, the voltage Ec, is applied to the rectifier D at a varying amplitude depending on the variations of pressure. This variation will correspondingly vary the rectified output represented by voltage ER, across resistor R3. This is indicated by the graph of Fig. 11 wherein voltage ER, is plotted against time. The wave marked ERF represents the half-wave envelope of the oscillator frequency, and the envelope of this wave marked ED represents the variation of the rectified voltage. The voltage Eo represents the rectified voltage developed at zero defiection of the diaphragm.

It will be recognized that the device and systems described and illustrated are subject to modification all within the scope of the invention. Many known additions and modifications may be made in the system, according to known practice. For example, filtering devices such as chokes and capacitors may be included in the amplifier following the rectified output to block the RF ripple voltage from the amplifier. Again, if desired, a balancing circuit may be included in this amplifier to balance out the steady state current corresponding to zero pressure which would otherwise be continuously applied to the indicator.

Another modification which might be used, if de-;

sired, is the use of a cathode follower or other known form of coupling system for feeding the indicator, which may be a galvanometer or other suitable indicating or recording device.

It will be recognized that by our invention we have succeeded in combining the desirable features of a condenser type of pickup with a sim-. plified electrical system, capable of measuring and transmitting rapidly varying pressure and other phenomena which can cause displacement of the condenser type pickup diaphragm.

Many advantages accrue from the use of our novel system. For example, it avoids the use of resonant circuits other than in the oscillator; Furthermore, the line between the pickup and: the oscillator, as Well as to the other apparatus connected with the pickup, are untuned and maybe of any desired length; for example, 500 or 1000 foot lines are completely feasible.

The pickup unit itself possesses many advan; tages of its own. Its simplified construction with the resistors and rectifier located in proximity to the condenser furnishes the desired simplicity. No built-in tuning coil is required such as has been used in other pickup devices; and all the essential elements of the pickup can be of a very small size. As a result, the pickup need be only a fraction of the size of formerly used pickup units. The frequency response of the system is the equal or superior of other known systems, being limited merely by the natural frequency of the condenser diaphragm and by the response of the recording system. Furthermore, our complete system has proven to possess much greater stability than other systems employing condenser-type pickups. This is largely a result of using an untuned line from oscillator to pickup with a resistor (R1) which not only terminates the line in its characteristic impedance but also acts as a ballast load to minimize the eifects of variations in line capacitance arising from temperature changes or vibration. A further factor of much importance which is an aid in producing high stability is the close proximity of the rectifier circuit elements to the variable condenser. It is thereby possible rigidly to tie down all leads, connections and circuit elements, thus avoiding the capacity changes usually arising within a pickup from vibration, shock and temperature changes.

We claim:

1. An electrical network having an input and an output and being adapted to transmit to an indicating means responsive to the output, signals from a source of oscillations connected across the input, which are modified in response to pressure variations received by a pickup of the type having a variable impedance element the impedance of which varies in accordance with said pressure variations, said network comprising a first resistance element shunted across said input, said variable impedance element and a second resistance element arranged in series with each other, the series arrangement being shunted across said first resistance element, a rectifier element and a third resistance element arranged in series with each other, said lastnamed series arrangement being shunted across said variable impedance element, said third resistance element being connected across said output, whereby D. C. voltage having variations of instantaneous value corresponding to the iinpedance variations of said variable impedance element is impressed across said output and received by the indicating means.

2. An electrical network having an input and an output and being adapted to transmit to an indicating means responsive to the output, si nals from a source of oscillations connected across the input, which are modified in response to pressure variations received by a pickup o! the type having a variable condenser element, the impedance of which varies in accordance with said pressure variations, said network comprising: a first resistance element shunted across said input, said variable condenser element and a second resistance elementarranged in series with each other. the series arrangement being shunted across said first resistance element. and a rectifier element arranged in series between said output and the junction of the second re sistance element and variable condenser element, whereby D, C. voltage having variations of instantaneous value corresponding to the impcdance variations of said variable condenser element is impressed across said output and received by said indicating means.

CHARLES M WOLFE. QUIDO M. SHULTISE. CLYDE W. GROW.

References Cited in the tile of this patent UNITED STATES PATENTS QTHER REFERENCES Book: Mechanical Measurements by Electrical Methods" by RobertsThc Instrument Publishing (70., Pittsburgh, Pa.., 1946, pages 2-30.